1. Field of the Invention
The present invention relates to an optical pickup for recording information signals on an optical disc and reproducing information signals from a recorded optical disc, and an optical disc apparatus including the optical pickup.
2. Description of the Related Art
As a recording medium of information signals, optical discs, such as CDs (compact discs) and DVDs (digital versatile discs), have been used, and there has been an optical disc apparatus for recording information signals on such an optical disc and reproducing information signals from a recorded optical disc. The optical disc apparatus is provided with an optical pickup that is moved in the radial direction of the optical disc to emit an optical beam on the optical disc.
The optical pickup generally includes a light source, a beam splitter, an objective lens, and a photo-receiver. An optical beam emitted from the light source passes through the beam splitter and is condensed by the objective lens to form an optical-beam spot on a recording layer of the optical disc. The optical beam condensed on the recording layer of the optical disc is also reflected to again enter the beam splitter so that its optical path is changed by the beam splitter, thereby entering the photo-receiver.
The optical disc includes a monolayer type having a single recording layer and a multi-layer type having a plurality of recording layers. In the multi-layer optical disc, when an optical beam is condensed on one recording layer, for example, the beam is reflected by other recording layers adjacent to the one recording layer.
In the monolayer optical disc, when an optical beam is condensed on the recording layer, the optical beam is also reflected by the surface of the optical disc (the other recording layers or surfaces are also referred to “other recording layers below”.
In such a manner, not only the optical beam reflected by the recording layer, on which the optical beam is condensed for recording/reproducing, but also the optical beam reflected by the other recording layers may be incident to the photo-receiver as stray light.
Such stray light may cause the quality deterioration of RF (radio frequency) signals and defects such as servo signal offset, as well as may cause the interference between the optical beams reflected the respective layers. Also, especially when an optical beam is divided with a diffraction element for producing a tracking-error signal, the interference between the principal light flux and the secondary light flux may pose a large problem due to a light density ratio.
In general, when an optical beam emitted from the light source is divided into 0-order light and ±1st-order diffracted light by a diffraction element, while the optical power of the 0-order light is increased as the principal light flux for detecting the RF signal, the ±1st-order diffracted light is about 10% of the 0-order light in the optical power in order to prevent the extinction of the recorded information on the recording layer due to the ±1st-order diffracted light as the secondary light flux.
If the 0-order light reflected by the other recording layers to be received on the photo-receiver herein is about 10% in the optical power per unit area of the 0-order light reflected by the recording layer where the optical beam is condensed for recording/reproducing information signals (also referred to as a “focus recording layer” below) to be received on the photo-receiver, for example, the ±1st-order diffracted light reflected by the focus recording layer to be received on the photo-receiver is to be roughly identical to the 0-order light reflected by the other recording layers to be received on the photo-receiver in the optical power, so that a great problem arises when a tracking error signal, spherical aberration, a land groove, and a crosstalk are detected using the ±1st-order diffracted light.
When information signals are recorded/reproduced on/from the optical disc having a plurality of recording layers in such a manner, the return light from the other layers adjacent to the focus recording layer exists in the photo-receiver other than return light from the focus recording layer for recording/reproducing, so as to interfere with each other. By the interference, a noise element may be unfavorably generated in the signal detected by a light detector.
In order to solve this problem, an optical pickup has been devised in that masking means having a masking member is arranged in the way back of an optical system for avoiding the interference by preventing the return light of the other recording layers from entering the photo-receiver (see Japanese Unexamined Patent Application Publication No. 2005-63595). However, the loss in the optical power is unfavorably increased.